This project is designed to determine if therapy for breast cancer induces certain predictable changes in the receptor content and, presumbaly, the clonal makeup of the tumor. With such information, it should be possible to design combinations of endocrine and other forms of therapy on a theoretical basis, instead of having to test each combination empirically. A secondary goal of this project is to evaluate a nuclear exchange assay for estrogen receptor as an adjunct for the current cytoplasmic assay in predicting response to endocrine therapy.